If the new wall should have been a party wall and relevant notifications submitted, can my brother pursue this or did this requirement die with the previous owner.
The OP raised a question about party walls and what happens when work is done without the relevant notices. it probably is within 3M of the property foundations and if it was a retaining wall it probably required excavation below the existing foundations. It might even be deep enough to be eligible under the 6M rule. Further notice will be needed if his permitted development foundations go below.
Nothing in the PWA stops you remedying "trespassing" foundations.